Reddit reviews MICTUNING LED Digital Double Voltmeter, Round Panel Voltage Monitor Blue for Car Pickup RV Truck Dual Battery Pack

First, You need to add up all the energy youre going to use in an average day. This is critical and no one else can really do it for you.

Theres a couple of ways to do this. You can buy a kill-a-watt meter and plug a power strip into it and then run everything you would want to run in a day off of it. This is it on amazon It will tell you how many watts everything has used.

Or you can go to a solar calculator on the web.....type in all the things you will use and what their rated wattages are. If you dont know them you can find them ..usually...on a label on the back near the power supply of each appliance. Or just google and use the larger number of their examples. Type in their wattage and how long you will use them. This too will tell you how many watts youre likely to use during a day.

Second, you need to size your solar array and your battery bank.

Lets say in your calculations you find that you use 2000w a day (2kw) that would be about 60kw a month if you want to compare it with your electric bill (which is pretty low but not unrealistic since you arent using a/c or any large appliances). For solar panels the math is pretty simple. If you get 4 hours of direct sunshine you would need 500w of solar panels to get your 2000w for the day under ideal conditions and assuming no loss. (never plan for ideal conditions and never assume zero loss but you get the idea) If you think youre going to get 8 (youre not) then you just need 250w....and so on.

You will probably never achieve this, I would shoot for 60% more solar on the bus than you think need on paper. For this example I would do my best to get 750 to 800 watts of panels. It is fair complicated and very build specific to try and calculate how much loss you will incur in your wiring and in panel placement. Since you can only have two panels the simplest and most elegant solution is to just buy panels that cover as much of the space as you have left as possible.

Panels also are never as efficient as they are the first year. If you size perfectly this year in a year or two you will be undersized.

For batteries you have to consider amp hours. Watts are amps * volts. Batteries are usually 12 v. Lets continue our example that you use 2000w a day and want to have enough reserve power to cover a full day. 2000w at 12v is 167 amps. An amp hour is one amp or one hour. We can take our 167 amps and know that you need 167 amp hours because youre using it over time. Im oversimplifying but thats the smallest amp/hour rating that will suffice. A good rule of thumb is to never drain lead acid batteries below 50% so now you need a 330 amp hours battery bank at the minimum. As an example that means you would need between 3 and 4 of these For lithium ion I think its 80% so thats 210 amp hours of lithium ion.

Your battery will never be as good as it is the first week so in a year's time neither of these banks will provide enough reserve energy if you just do the minimum required right off the bat. You will have to overbuild to account for this or add later.

If you want to work backwards then its a little less elegant. Lets say your coffee maker is 5 amps but its rated at 110 V thats 550 watts. Lets assume you use it for 15 minutes so thats ~138 watt hours. Your inverter will need to draw (138 watts divided by 12 V) about 11.5 amp hours out of your battery. If you have a 100 ah battery bank, that gives you 50 ah effective use...that 15 minutes of coffee making just used over 20 percent of your battery.

Charging your house batteries off the alternator can be very simple. This is what I used Put a switch in the cab and run a wire to the small terminal. When the small pole is energized it will connect the two larger poles. Wire one pole to one battery bank and the other pole to the other battery bank, use thick wire for the large poles 2/0. When the small pole is not energized the two poles will not be connected. When you want one bank to charge the other bank turn your switch on. When you want one bank to not drain the other bank, turn your switch off.

Im partial to this brand but you may find a cheaper one.

There are dozens of gauges that will tell you your battery voltages. Here is one example if you have everything wired correctly when you flip your switch to connect your two battery banks you should see their voltages come together.

Sort of. There's sort of a progression going on. It's a good idea to ensure you can cut the power between the two manually as a safeguard.

I used this
last time with a fuse on each battery, and just kept the accessories live to the house battery full-time. Turned it on when I wanted to charge or the odd case of boosting my starter battery from the house. Dead-simple hook up, I just had to pay attention to when I should have it on or not. Mostly cut it on before I start the vehicle, watch the dash voltmeter to know when everything's charged, then cut it off before I start using the house without the motor running.

I'm getting this one next time to replace the one above. I'll have the common on the accessories, and be able to run them off of the starter, the house, or both. "both" would also be used to charge the house off the alternator, and I'll have it there almost all the time, with the addition of a VSR (below). It also has an "off", in which nothing is connected to anything. You'd still want to wait until the engine is running to re-charge a dead house battery, and switch it to only "2" to keep from draining the starter battery, unless you install one of the next two items.

This makes a nice addition wired right alongside either switch. Tells you how much juice each battery has.

To keep everything 100% un-screw-up-able, this is going to go between my starter battery positive and my new manual isolator switch in back. If either side gets to charging voltage, it will connect to the other side. That means the alternator will always charge the starter battery, but also charge the house battery AND if I connect a charger to the house battery (like a generator or plugging in to shore; solar that fits on a van would take several days to charge a battery from dead to van-crankable), then it will automatically charge and maintain the starter battery, too. Neither can drain the other past the point it wouldn't crank the van.

Charging relays are much cheaper than a VSR, and are automatic in a more rudimentary way. This will do the job fine for most cases, and is a little less expensive than a VSR. The big lugs on it hook up just like the two on the VSR above, except it's not automatic - you hook the small poles up to stuff to tell it when to connect the big lugs. The small positive goes to the other, smaller wire going into the alternator, only allowing it to connect if the alternator is charging. The small negative can go straight to the large negative, but I'd run it through a small switch in the dashboard somewhere (and the other side of that switch back to the large negative lug) to allow me to disable charging house battery off the alternator. it's a decent blend of manual and automatic control, with a relatively tiny price tag. This pretty much needs to go under the hood, though, whereas the VSR could actually go in the power center or under the hood. Remember that either would go on the 4 gauge between starter battery and rear isolator, if present; it's the longest fat-wire in the whole system. some people skip the manual isolator switch altogether, and just go with a VSR or charging relay.

Only because I happen to have one of the first switches linked here already, I'd install it between the large lugs of the VSR or charging relay, to bypass it manually. If I didn't already have one, I'd just count on using a set of jumper cables to do that in the rare case I should need to turn that switch on.